1. Field of the Invention
The present invention relates to a speaker, and more particularly, to a speaker of a motional-feedback type, which detects vibrations of a vibration system and feeds back a detection signal to a drive system.
2. Description of the Related Art
Reproduction capability of a low-pitched frequency sound of a typical speaker greatly depends on the lowest resonance frequency specific to the speaker. For instance, as an area of a diaphragm increases, the lowest resonance frequency decreases by contrary, thus leading to improvement in the reproduction capability of a low-pitched frequency sound. However, an increase of its diaphragm size results in not only an increase in the overall dimension of the speaker but also a gain of weight of the diaphragm, which lowers efficiency of the speaker. In order to solve such problems, a speaker has been developed, in which the so-called motional feedback (MFB) is adopted for detecting amplitude of a diaphragm and feeding back a detection signal to an input signal.
The speaker adopted the MFB, in which a tabular piezoelectric element is provided on the top of a voice coil bobbin through a supporting member. In this case, the voice coil bobbin is driven by a signal input to a voice coil, the diaphragm vibrates by the aid of vibrations of the bobbin, thereby producing a sound. Simultaneously, the tabular piezoelectric element also vibrates through the supporting member, and an output detected being obtained depending on how much the diaphragm vibrates. Alternatively, a top plate is fitted on the top of the voice coil bobbin, a supporter is stood up on the center of the top plate, and the center of the tabular piezoelectric element is adhered on the top of the supporter. In addition, this tabular piezoelectric element is supported only by the supporter serving as a supporting member.
As an example, see Japanese Patent Publication Laid-Open No. 57-119596 (Page 2, Col Upper Left, Lines 5–11, Col Upper Right, line 17 to Col Lower left, line 1. FIG. 2 and FIG. 5).
The conventional speaker adopted the motional feedback has, on one hand that, the advantage that reproduction capability of a low-pitched frequency sound can be improved by electrically compensating the lowest resonance frequency without inviting an increase in its diaphragm size. The speaker has, on the other hand, the disadvantage that efficiency of the speaker lowers because weights of the top plate, supporting member (supporter), and tabular piezoelectric element are collectively applied to the diaphragm, which gains the weight of the vibration system. Moreover, the speaker also has the disadvantage that it shows poor workability on production as the tabular piezoelectric element is installed on the diaphragm through the top plate and the supporting member (supporter). In addition, the speaker also has the disadvantage that it presents low maintainability since the tabular piezoelectric element has been integrally combined with the diaphragm, resulting in the shortcoming that the diaphragm is under the pressure of necessity to have to replace the diaphragm as well with new one, when the tabular piezoelectric element gets out of order.